Electro-optic displays, and processes for the production thereof

ABSTRACT

Improvements in the production of electro-optic displays include: (a) use of a masking film to keep a selected area of a backplane (such as a front electrode contact) free from electro-optic material; (b) spray coating of electrophoretic capsules on to a substrate under controlled conditions; (c) forming a monolayer of capsules on a substrate by prior deposition of a water-swellable polymer; and (d) overcoating a layer of electro-optic material with a solvent-free polymerizable liquid material, contacting this layer with a light-transmissive electrode layer, and polymerizing the liquid material to adhere the electrode layer to the electro-optic material.

REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No.14/989,031, filed Jan. 6, 2016, claims benefit of provisionalApplication Ser. No. 62/100,950, filed Jan. 8, 2015.

This application is related to U.S. Pat. Nos. 6,982,178; 7,561,324; and7,839,564. The entire contents of these patents, and of all other U.S.patents and published and copending applications mentioned below, areherein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to electro-optic displays and processes for theproduction thereof. More specifically, this invention relates toprocesses for the production of electro-optic displays without the useof front plane laminates, inverted front plane laminates and doublerelease films as described in the aforementioned U.S. Pat. Nos.6,982,178; 7,561,324; and 7,839,564, and to processes for depositingencapsulated electrophoretic media by spraying.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable. [Para 12] Another type of electro-optic displayis an electro-wetting display developed by Philips and described inHayes, R. A., et al., “Video-Speed Electronic Paper Based onElectrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat.No. 7,420,549 that such electro-wetting displays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728; and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276; and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,825,829; 6,982,178; 7,236,292;        7,443,571; 7,513,813; 7,561,324; 7,636,191; 7,649,666;        7,728,811; 7,729,039; 7,791,782; 7,839,564; 7,843,621;        7,843,624; 8,034,209; 8,068,272; 8,077,381; 8,177,942;        8,390,301; 8,482,852; 8,786,929; 8,830,553; 8,854,721; and        9,075,280; and U.S. Patent Applications Publication Nos.        2009/0109519; 2009/0168067; 2011/0164301; 2014/0027044;        2014/0115884; and 2014/0340738;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564;        6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790;        6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438;        6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578;        6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519;        6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769;        6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640;        6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296;        7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752;        7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751;        7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148;        7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572;        7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712;        7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674;        7,667,886; 7,672,040; 7,688,497; 7,733,335; 7,785,988;        7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053;        7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141;        8,089,453; 8,208,193; 8,373,211; 8,389,381; 8,498,042;        8,610,988; 8,728,266; 8,754,859; 8,830,560; 8,891,155;        8,989,886; 9,152,003; and 9,152,004; and U.S. Patent        Applications Publication Nos. 2002/0060321; 2004/0105036;        2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489;        2007/0109219; 2009/0122389; 2009/0315044; 2011/0026101;        2011/0140744; 2011/0187683; 2011/0187689; 2011/0292319;        2013/0278900; 2014/0078024; 2014/0139501; 2014/0300837;        2015/0171112; 2015/0205178; 2015/0226986; 2015/0227018;        2015/0228666; and 2015/0261057; and International Application        Publication No. WO 00/38000; European Patents Nos. 1,099,207 B 1        and 1,145,072 B 1;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 7,075,502; and 7,839,564;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600; and 7,453,445;    -   (g) Applications of displays; see for example U.S. Pat. Nos.        7,312,784; and 8,009,348; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549; 8,319,759; and 8,994,705; and        U.S. Patent Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic materials may also be used in the presentinvention.

An electro-optic display normally comprises a layer of electro-opticmaterial and at least two other layers disposed on opposed sides of theelectro-optic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electro-optic display, which is intended foruse with a stylus, print head or similar movable electrode separate fromthe display, only one of the layers adjacent the electro-optic layercomprises an electrode, the layer on the opposed side of theelectro-optic layer typically being a protective layer intended toprevent the movable electrode damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts asone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive. Similar manufacturingtechniques can be used with other types of electro-optic displays

As discussed in the aforementioned U.S. Pat. No. 6,982,178, (see column3, lines 63 to column 5, line 46) many of the components used in solidelectro-optic displays, and the methods used to manufacture suchdisplays, are derived from technology used in liquid crystal displays(LCD's), which are of course also electro-optic displays, though using aliquid rather than a solid medium. For example, solid electro-opticdisplays may make use of an active matrix backplane comprising an arrayof transistors or diodes and a corresponding array of pixel electrodes,and a “continuous” front electrode (in the sense of an electrode whichextends over multiple pixels and typically the whole display) on atransparent substrate, these components being essentially the same as inLCD's. However, the methods used for assembling LCD's cannot be usedwith solid electro-optic displays. LCD's are normally assembled byforming the backplane and front electrode on separate glass substrates,then adhesively securing these components together leaving a smallaperture between them, placing the resultant assembly under vacuum, andimmersing the assembly in a bath of the liquid crystal, so that theliquid crystal flows through the aperture between the backplane and thefront electrode. Finally, with the liquid crystal in place, the apertureis sealed to provide the final display.

This LCD assembly process cannot readily be transferred to solidelectro-optic displays. Because the electro-optic material is solid, itmust be present between the backplane and the front electrode beforethese two integers are secured to each other. Furthermore, in contrastto a liquid crystal material, which is simply placed between the frontelectrode and the backplane without being attached to either, a solidelectro-optic medium normally needs to be secured to both; in most casesthe solid electro-optic medium is formed on the front electrode, sincethis is generally easier than forming the medium on thecircuitry-containing backplane, and the front electrode/electro-opticmedium combination is then laminated to the backplane, typically bycovering the entire surface of the electro-optic medium with an adhesiveand laminating under heat, pressure and possibly vacuum. Accordingly,most prior art methods for final lamination of solid electrophoreticdisplays are essentially batch methods in which (typically) theelectro-optic medium, a lamination adhesive and a backplane are broughttogether immediately prior to final assembly, and it is desirable toprovide methods better adapted for mass production.

Electro-optic displays are often costly; for example, the cost of thecolor LCD found in a portable computer is typically a substantialfraction of the entire cost of the computer. As the use of electro-opticdisplays spreads to devices, such as cellular telephones and personaldigital assistants (PDA's), much less costly than portable computers,there is great pressure to reduce the costs of such displays. Theability to form layers of some solid electro-optic media by printingtechniques on flexible substrates, as discussed above, opens up thepossibility of reducing the cost of electro-optic components of displaysby using mass production techniques such as roll-to-roll coating usingcommercial equipment used for the production of coated papers, polymericfilms and similar media.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term “light-transmissive” should ofcourse be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly(ethylene terephthalate) (PET) films coated withaluminum or ITO are available commercially, for example as “aluminizedMylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours& Company, Wilmington Del., and such commercial materials may be usedwith good results in the front plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a method fortesting the electro-optic medium in a front plane laminate prior toincorporation of the front plane laminate into a display. In thistesting method, the release sheet is provided with an electricallyconductive layer, and a voltage sufficient to change the optical stateof the electro-optic medium is applied between this electricallyconductive layer and the electrically conductive layer on the opposedside of the electro-optic medium. Observation of the electro-opticmedium will then reveal any faults in the medium, thus avoidinglaminating faulty electro-optic medium into a display, with theresultant cost of scrapping the entire display, not merely the faultyfront plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a secondmethod for testing the electro-optic medium in a front plane laminate byplacing an electrostatic charge on the release sheet, thus forming animage on the electro-optic medium. This image is then observed in thesame way as before to detect any faults in the electro-optic medium.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

The aforementioned U.S. Pat. No. 7,561,324 describes a so-called “doublerelease sheet” which is essentially a simplified version of the frontplane laminate of the aforementioned U.S. Pat. No. 6,982,178. One formof the double release sheet comprises a layer of a solid electro-opticmedium sandwiched between two adhesive layers, one or both of theadhesive layers being covered by a release sheet. Another form of thedouble release sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

The aforementioned U.S. Pat. No. 7,839,564 describes a so-called“inverted front plane laminate”, which is a variant of the front planelaminate described in the aforementioned U.S. Pat. No. 6,982,178. Thisinverted front plane laminate comprises, in order, at least one of alight-transmissive protective layer and a light-transmissiveelectrically-conductive layer; an adhesive layer; a layer of a solidelectro-optic medium; and a release sheet. This inverted front planelaminate is used to form an electro-optic display having a layer oflamination adhesive between the electro-optic layer and the frontelectrode or front substrate; a second, typically thin layer of adhesivemay or may not be present between the electro-optic layer and abackplane. Such electro-optic displays can combine good resolution withgood low temperature performance.

As already indicated, the aforementioned front plane laminates, invertedfront plane laminates and double release films are well adapted forproduction by roll-to-roll processes, thus producing the front planelaminate, inverted front plane laminate or double release film in theform of a roll of material which can be severed into pieces of the sizeneeded for individual displays and laminated to appropriate backplanes.However, also as already indicated, to effect the necessary lamination,and layer of lamination adhesive normally needs to be present betweenthe electro-optic layer itself and the backplane, and this layer oflamination adhesive remains in the final display between the twoelectrodes. The presence of this lamination adhesive layer hassignificant effects on the electro-optic properties of the display.Inevitably, some of the voltage drop between the electrodes occurswithin the lamination adhesive layer, thus reducing the voltageavailable for driving the electro-optic layer. The effect of thelamination adhesive tends to become greater at lower temperatures, andthis variation in the effect of lamination adhesive with temperaturecomplicates the driving of the display. The voltage drop within thelamination adhesive can be reduced, and the low temperature operation ofthe display improved, by increasing the conductivity of the laminationadhesive layer, for example by doping the layer with tetrabutylammoniumhexafluorophosphate or other materials as described in U.S. Pat. Nos.7,012,735 and 7,173,752. However, increasing the conductivity of thelamination adhesive layer in this manner tends to increase pixelblooming (a phenomenon whereby the area of the electro-optic layer whichchanges optical state in response to change of voltage at a pixelelectrode is larger than the pixel electrode itself), and this bloomingtends to reduce the resolution of the display. Hence, this type ofdisplay apparently intrinsically requires a compromise between lowtemperature performance and display resolution.

One aspect of the present invention relates to processes for theproduction of electro-optic displays which do not require the presenceof a lamination adhesive layer between the electro-optic layer and thebackplane; these processes involve coating the electro-optic material onto the backplane.

A second aspect of the present invention relates to novel processes forapplication of encapsulated electrophoretic media to substrates. Theseprocesses may be used to aid in the first aspect of the invention butmay also be used in other types of coating processes.

The electrophoretic media described in the aforementioned E Ink patentsand applications, and similar prior art electrophoretic media typicallycomprise electrophoretic particles, charge control agents, imagestability agents and flocculants in a non-polar liquid, typicallyencapsulated in a flexible organic matrix such as a gelatin/acaciacoacervate. To produce commercial displays, it is necessary to coat athin layer (preferably a monolayer—see U.S. Pat. No. 6,839,158) ofcapsules on a substrate, which may be a front substrate bearing anelectrode (see the aforementioned U.S. Pat. No. 6,982,178), a backplaneor a release sheet. Hitherto, coating of encapsulated electrophoreticmedia on substrates has typically been effected by slot coating, inwhich a slurry of capsules in a carrier medium is forced through a sloton to a substrate which is moving relative to the slot. Slot coatingimposes limitations upon the viscosity and other physical properties ofthe material being coated and typically requires the addition of slotcoating additives to control the rheology of the coated material toensure that the coating does not flow and develop non-uniformities inthickness prior to drying. Thus, in slot coating electrophoreticcapsules are typically supplied in the form of aqueous slurriescontaining optional latex binder, rheology modification agents, ionicdopants, and surfactants. These additives remain in the final driedelectrophoretic medium and may affect its properties, including itselectro-optic properties.

Furthermore, although slot coating is well adapted for applyingelectrophoretic media to continuous webs, it is not well adapted for“patch” coating of discrete areas of a web or discrete parts (forexample, individual backplanes) lying on a moving belt, since settlingand self-segregation of capsule slurry within the slot die manifoldbecome problematic during such “interrupted” capsule depositionprocesses. Slot coating is generally not useful for non-planarsubstrates, which is unfortunate since encapsulated electrophoreticmedia are well adapted for coating three-dimensional objects, includingarchitectural features. Other problems with slot coating includechatter-like streaks parallel to the coating slot die (these streaks arebelieved to result from periodic bunching or jamming of capsules), andstreaking in the direction of coating (believed to be due to capsulejamming or non-uniform flows in delivery of capsules to the slot coatingslot die).

The aforementioned problems with slot coating have resulted in a searchfor an alternative coating technology able to cope with patch coatingand coating of non-planar substrates, as well as planar objects andwebs. One well established coating technology which has been consideredfor this purpose is spray coating, i.e., the pneumatic atomization anddeposition of capsule dispersions. Spray coating is a mature technology,but prior art attempts to apply the technology to capsule depositionhave been subject to various defects and modes of failure. Because theytypically have flexible capsule walls, capsules deform and sometimerupture during spraying, either during the atomization step or uponimpact on the target. The consequences of significant capsule rupture,including the release of electrophoretic particles, fluid etc., are sosevere that, so far as the present inventors are aware, unacceptablelevels of ruptured capsules have by themselves been sufficient to doomall previous attempts to spray coat encapsulated electrophoretic media.The second aspect of the present invention provides a spray coatingprocess which reduces or eliminates these problems.

A third aspect of the present invention relates to processes forreducing the adhesion of capsules to a substrate during coating in orderto facilitate close packing of capsules on the substrate. This adhesionreduction process is primarily intended for use with spray coating ofcapsules but may also be useful with other capsule depositiontechniques.

As previously mentioned, in the production of electrophoretic displaysit is generally preferred to form a monolayer of capsules on asubstrate. However, a common problem encountered when coatingelectrophoretic capsules on to a substrate (typically a ITO/PET film, aPET/release film, or any type of silicone release film) is that thecapsules adhere strongly to the substrate and are unable to rearrangethemselves into an optimally packed monolayer upon drying. Variouscoating materials have been found to significantly reducecapsule-substrate adhesion, thus allowing the capsules to rearrangethemselves by means of capillary forces during drying. Unfortunately, ifsuch coating materials are used in slot coating processes employing adoctor blade, as is common during slot coating, the reducedcapsule-substrate adhesion causes the capsules to not pass properly pastthe doctor blade; instead, the vast majority of the capsules are simplypushed in front of the doctor blade, leaving only a very sparse capsulecoating on the substrate. Accordingly, there is a need for an improvedprocess for the formation of closely packed monolayers of capsules onsubstrates, and the third aspect of the present invention seeks toprovide such a process.

A fourth aspect of the present invention relates to processes forovercoating electro-optic materials to planarize an electro-optic layerand/or adhere the electro-optic layer to a transparent front electrodethat may be attached to a color filter.

It is known (see especially U.S. Pat. No. 7,839,564) that a colordisplay may be formed by overlaying a color filter array (CFA) over anmonochromatic black/white electro-optic display, with the CFA elementsaligned with the pixel electrodes of the backplane. Such a CFA may forexample have repeating red, green and blue stripes, or a repeating 2×2red/green/blue/white (clear) pixel pattern. The brightest state of sucha display is achieved when all pixels of the electro-optic layer arewhite, and it is therefore preferred that the absorption of the CFAelements, taken as a whole, be constant across the visible range, sothat the brightest state will have no color tint.

Overlaying a CFA over an electro-optic layer in this manner leads to atrade-off between brightness and color saturation, and the colors thatare most difficult to render are the brightest colors, such as white andyellow. Moreover, such a display suffers from several sources of lightloss or contamination that limit still further the quality of colorattainable. These include:

-   -   (a) absorption of light by the white state of the electro-optic        layer that limits the brightness of all colors; this may be as        much as 50% of light incident on a white region of the        electro-optic layer;    -   (b) reflection of light by the dark state of the electro-optic        layer, causing pollution of a desired color by unwanted light of        other colors;    -   (c) contrast at the pixel level (“local contrast”) may be lower        than the contrast measured if the entire display is switched        from the white state to the dark state, due to electrical        effects at the edges of pixels (i.e., image “blooming”) or to        optical effects related to the scattering length within the        electro-optic layer (i.e., “optical dot gain”);    -   (d) loss of light due to total internal reflection within the        display; since electrophoretic and most other reflective        electro-optic layer are Lambertian reflectors, a significant        proportion of light may be reflected at angles to the normal        greater than the critical angle for total internal reflection at        at least one surface between adjacent layers of the display and        be lost;    -   (e) illumination parallax: if the CFA elements are significantly        separated from the electro-optic layer by intervening layers,        light incident on the display at sufficiently large angles to        the normal may pass through a color filter element of one color        and exit the display through an element of a different color,        leading to pollution of the colored image and a color shift; and    -   (f) viewing parallax: for the same reasons as in (e), if a        viewer observes the display at a sufficiently large angle to the        normal, and the CFA elements are significantly separated from        the electro-optic layer, the viewer may see modulation of        reflectivity through an unintended color filter element.

When an electro-optic display is formed using a front plane laminate, asdescribed above with reference to U.S. Pat. No. 6,982,178, a singleadhesive layer is present between the electro-optic layer and thebackplane. Although this adhesive layer is not disposed between theelectro-optic layer and the CFA (and thus does not contribute to most ofthe problems discussed above), it is present between the electrodes ofthe display, and thus contributes to image blooming. The presence ofthis adhesive layer also diminishes the voltage drop actually occurringacross the electro-optic layer, which tends to limit the reflectivity ofthe white state of the electro-optic layer and its contrast ratio. Whenan electro-optic display is formed using either a double release film,as described above with reference to U.S. Pat. No. 7,561,324, or aninverted front plane laminate, as described above with reference to U.S.Pat. No. 7,839,564, typically two adhesive layers will be present, thefirst between the CFA and the electro-optic layer, and the secondbetween the electro-optic layer and the backplane. The second adhesivelayer contributes to the same problems as the adhesive layer derivedfrom an FPL, as already discussed; the first adhesive layer at leastcontributes to the illumination and viewing parallax problems, and mayalso contribute to the total internal reflection problem.

There is thus a need for a process for producing electro-optic displayswhich reduces or eliminates the problems caused by the presence ofadhesive layers between the electrodes. However, since as discussedabove, manufacture of electro-optic displays necessitates at least onelamination operation, the best process will involve the provision ofonly one thin adhesive layer, and the present invention seeks to providesuch a process.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides a process forproducing a sub-assembly for use in an electro-optic display, theprocess comprising:

-   -   providing a backplane comprising at least one electrode located        in a first area of the backplane;    -   covering a second area of the backplane spaced from the        electrode with a masking layer;    -   coating the backplane having the masking layer thereon with a        layer of an electro-optic material; and    -   removing the masking layer, and the portion of the layer of        electro-optic medium thereon, from the backplane, thereby        producing a sub-assembly comprising the backplane having its        first area covered by the layer of electro-optic material but        its second area free from the layer of electro-optic material.

This process may hereinafter for convenience be referred to as the“masked backplane” process of the invention. In one form of thisprocess, the second area of the backplane comprises a contact padintended, in the final display, to make electrical contact with thefront electrode of the display. In this form of the invention, alight-transmissive conductive layer may be partially covered with alamination adhesive, and laminated to the backplane/electro-opticmaterial sub-assembly with the lamination adhesive contacting theelectro-optic material and the light-transmissive conductive layer inelectrical contact with the contact on the backplane. This electricalcontact may be direct, or may be made via an electrically conductive inkor similar deformable conductive material.

In another form of the process, after the layer of electro-opticmaterial has been coated on the backplane, but before the masking layeris removed, a layer of lamination adhesive (preferably a 100 per centsolids radiation-curable adhesive) is coated over the electro-opticmaterial, and then the masking layer is removed, together with both theportions of the layers of electro-optic material and the laminationadhesive thereon. A light-transmissive electrically-conductive layer canthen be laminated to the lamination adhesive in the first area of thebackplane; the light-transmissive electrically-conductive layerpreferably extends into the second area of the backplane so as to makeelectrical contact with the contact pad in this second area.

In a further form of the process, the backplane has a third area whichis covered by a second masking layer which can be removed separatelyfrom the (first) masking layer covering the second area. In this form ofthe process, after the layer of electro-optic material has been coated,the second masking layer is removed, thus exposing the third area of thebackplane. A light-transmissive electrically-conductive layer is nowcoated over the backplane.

In a second aspect, this invention provides a process for sprayingcapsules of an electrophoretic medium on to a substrate, the processcomprising:

-   -   forming a dispersion of the capsules in a liquid;    -   feeding the dispersion through a first orifice; and    -   feeding a continuous stream of gas through a second, annular        orifice surrounding the first orifice, thereby forming a spray        of the capsules.

This process may hereinafter for convenience be referred to as the“spray coating process” of the present invention. The process mayinclude shaping the spray by feeding a continuous stream of gas througha plurality of shaping orifices disposed adjacent the spray. The spraymay be directed on to any type of substrate, including a web, aplurality of discrete objects disposed on a support or one or morethree-dimensional (i.e., non-planar) objects. If, as is typically thecase, the capsule walls are formed from a hydrophilic material (such asthe aforementioned gelatin/acacia coacervate), the liquid used todisperse the capsules is desirably aqueous; depending upon the specificcapsules and liquid used, the liquid may optionally comprise any one ormore of pH modifiers, surfactants and ionic dopants. The gas passedthrough both the second orifice and the shaping orifices is typicallyair, but it some cases it may be useful to use an inert gas, for examplenitrogen.

The spray coating process of the present invention may include the useof a masking material covering part of the substrate so that, afterremoval of the masking material, capsules remain only on those portionsof the substrate where the masking material was not present. Such a“masked spray coating process” of the invention may comprise multiplesteps with each step involving the use of a different mask and adifferent capsule dispersion so that the different capsule dispersionsare disposed in different areas of the final display.

In a third aspect, this invention provides a process for forming amonolayer of capsules on a substrate, the process comprising:

-   -   depositing a solution of a water-swellable polymer on the        substrate; and thereafter depositing a quantity of the capsules        sufficient to form a monolayer of capsules on to the substrate,        and allowing the capsules to arrange themselves into a monolayer        on the substrate.

This process may hereinafter for convenience be referred to as the“swellable polymer coating process” of the present invention. Thepolymer used may be, for example, a polysaccharide, such as a pectin, ora protein, especially an albumin. The albumin used may be, for example,egg albumin or bovine serum albumin; other types of albumin may also besuitable. At least when the capsules are formed of a hydrophilicmaterial (such as the gelatin/acacia coacervate mentioned in many of theE Ink patents and applications mentioned above), the polymer solution ispreferably an aqueous solution. The deposition of the capsules isdesirably effected by a process such as spray coating which does notrequire contact of a coating head or coating bar with the capsule layer.

In a fourth aspect, this invention provides a process for forming anelectro-optic display, the process comprising:

-   -   providing a backplane comprising at least one electrode;    -   coating the backplane with a layer of an electro-optic material;    -   depositing a layer of a substantially solvent-free polymerizable        liquid material over the layer of electro-optic material;    -   contacting the polymerizable liquid material with at least one        light-transmissive electrically-conductive layer; and    -   exposing the polymerizable liquid material to conditions        effective to cause polymerization of the material, thereby        adhering the at least one light-transmissive        electrically-conductive layer to the layer of electro-optic        material.

This process may hereinafter for convenience be referred to as the“overcoat layer process” of the present invention. In this process, thecoating of the backplane with the electro-optic material may beconducted by the masked backplane process of the present invention (topermit masking of any areas, for example row and column electrodes,which should not be covered by electro-optic material) and/or by thespray coating process of the present invention.

In one form of the overcoat layer process of the present invention, botha light-transmissive electrically-conductive layer and a color filterarray are adhered to the electro-optic layer. Typically, thelight-transmissive electrically-conductive layer is mounted on a frontsubstrate, which is provided with a color filter array (which may beprinted directly on to the front substrate). The front substrate mayserve to provide mechanical support to the electrically-conductivelayer; many conductive layers, for example indium-tin-oxide (ITO) layersare too fragile to be self-supporting.

In a second form of the overcoat layer process of the present invention,the electro-optic layer is a color electro-optic layer capable ofdisplaying a variety of colors (a so-called “inherent color” layer), andonly a front plane electrode (and any supporting substrate required) areadhered to the electro-optic layer by means of the polymerizable liquidmaterial.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 of the accompanying drawings is a schematic top plan view of abackplane undergoing a first masked backplane process of the presentinvention.

FIG. 2A is a schematic section along line 2A-2A in FIG. 1 but showingthe backplane after deposition of electro-optic material thereon.

FIG. 2B is a schematic section similar to that of FIG. 2A but showingthe backplane after the masking layer has been removed.

FIG. 2C is a schematic cross-section through a sub-assembly comprising alight-transmissive, electrically-conductive layer and a layer oflamination adhesive.

FIG. 2D is a schematic section similar to those of FIGS. 2A and 2B butshowing the backplane after lamination to the backplane of thesub-assembly shown in FIG. 2C via the layer of lamination adhesive.

FIG. 3A is a schematic section similar to that of FIG. 2A but showingthe same stage in a second masked backplane process of the presentinvention.

FIG. 3B is a schematic section similar to those of FIGS. 2B and 3A butshowing the backplane after formation of a layer of lamination adhesiveoverlying the electro-optic material but before removal of the maskinglayer.

FIG. 3C is a schematic section similar to that of FIG. 3B but showingthe backplane after removal of the masking layer.

FIG. 3D is a schematic section similar to that of FIGS. 2D and 3C butshowing the backplane after lamination of a front electrode and frontsubstrate to the electro-optic material via the layer of laminationadhesive.

FIG. 4A is a schematic section similar to those of FIGS. 2A and 3A butshowing the same stage in a third masked backplane process of thepresent invention which uses two separate masking layers.

FIG. 4B is a schematic section similar to that of FIG. 4A but showingthe backplane after removal of the second masking layer.

FIG. 4C is a schematic section similar to those of FIGS. 4A and 4B butshowing the backplane after formation of a light-transmissive frontelectrode layer overlying the electro-optic material.

FIG. 4D is a schematic section similar to those of FIGS. 4A-4C butshowing the backplane after removal of the first masking layer.

FIG. 5 is a schematic top plan view, similar to that of FIG. 1, of thebackplane used in the third masked backplane process after applicationof the two masking layers but before deposition of the electro-opticlayer.

FIG. 6 is a schematic cross-section through a simple spray coatingnozzle used in the spray coating process of the present invention.

FIG. 7 is a cross-section through a more complex spray coating nozzleused in the spray coating process of the present invention.

FIG. 8 is an enlarged view of the portion of the nozzle lying within thedotted box in FIG. 7.

FIG. 9 is a graph illustrating preferred parameters used in the spraycoating process of the present invention.

FIG. 10 is a top plan view of a first electrophoretic display producedby a masked spray coating process of the present invention.

FIG. 11 is a top plan view of a second electrophoretic display producedby a masked spray coating process of the present invention.

FIGS. 12A-12H are time lapse micrographs showing an experimentaldemonstration of the swellable polymer coating process of the presentinvention using capsules deposited from a pipette on to a microscopeslide.

FIG. 13 is a schematic cross-section through a prior art color displayusing a color filter array and produced using a double release film aspreviously described.

FIG. 14 is a schematic cross-section similar to that of FIG. 13 buttaken through a color display produced by the overcoat layer process ofthe present invention.

FIG. 15 is a schematic cross-section similar to those of FIGS. 13 and 14through a prior art inherent color display.

FIG. 16 is a schematic cross-section similar to those of FIGS. 13 to 15but taken through an inherent color display produced by the overcoatlayer process of the present invention.

DETAILED DESCRIPTION

As already indicated, this invention has several different aspects,which will primarily be described separately below. However, it shouldbe appreciated that a single physical process may make use of more thanone aspect of the invention. Thus, for example, a masked backplaneprocess of the invention might make use of the air spray coating processof the invention to deposit electro-optic material on a backplanetreated by the swellable coating process of the invention, and the layerof electro-optic material might thereafter be treated by the overcoatlayer process of the invention.

Masked Backplane Process

As indicated above, the masked backplane process of the presentinvention provides a process for producing a sub-assembly for use in anelectro-optic display. This process comprises providing a backplanecomprising at least one electrode located in a first area of thebackplane; covering a second area of the backplane spaced from theelectrode with a masking layer; coating the backplane having the maskinglayer thereon with a layer of an electro-optic material; and removingthe masking layer, and the portion of the layer of electro-optic mediumthereon, from the backplane, thereby producing a sub-assembly comprisingthe backplane having its first area covered by the layer ofelectro-optic material but its second area free from the layer ofelectro-optic material. Thus, the masked backplane process allows forformation of a sub-assembly, and ultimately an electro-optic display,without the use of a pre-formed front plane laminate and without thepresence of a lamination adhesive layer between the electro-optic layerand the backplane, thus reducing or eliminating the problems associatedwith this lamination adhesive layer, as discussed above.

The masked backplane process of the present invention builds thesub-assembly or display starting from the backplane. The process may becarried out on individual backplanes but for mass production purposes itis more conveniently effected on backplanes arranged in a multi-upconfiguration. Conceptually, when used to produce a complete display,the masked backplane process may be regarded as involving (a) thebackplane itself, which may be a segmented, passive matrix or activematrix backplane; (b) a masking layer for protecting areas of thebackplane which are not be covered by the electro-optic material (theprotected areas will typically be those required for making a “top planeconnection” to the front electrode of the final display, and may alsoinclude areas used for conductors leading to the pixel electrodes andelectronic components such as row and column drivers); (c) a solidelectro-optic medium, typically an encapsulated electrophoretic medium;(d) a light-transmissive, electrically-conductive layer which forms thefront electrode of the display; and (e) a means, typically a laminationadhesive, for securing the light-transmissive, electrically-conductivelayer to the solid electro-optic medium.

As already indicated, the backplane used in the masked backplane processmay be of any known type, although care should be taken to ensure thatthe backplane used does not damage the electro-optic layer. For exampleif the electro-optic layer is to be formed from an encapsulatedelectrophoretic medium, the backplane should not have such sharp changesin level as to risk puncturing some of the capsules. The masking layermay be formed from a simple polymeric film which adheres to thebackplane either because of its own physical properties or with the aidof an adhesive coating, but should desirably not be more than about 75μm in thickness; polymeric films which have been found useful as maskinglayers include Kapton tape (a polyimide tape available from du Pont deNemours & Company, Wilmington, Del.) and RP301 film (an acrylic filmavailable form Nitto America, Inc., Fremont Calif.). The solidelectro-optic layer is typically an encapsulated electrophoretic layerbut may also be a polymer-dispersed electrophoretic layer or a rotatingbichromal member or electrochromic layer. Care should be taken to ensurethat the physical properties of the electro-optic layer are such thatthe portions of the layer overlying the masking layer are removed whenthe masking layer itself is removed, without tearing the masking layerso that portions of the masking layer are left on the backplane and/orwithout portions of the electro-optic layer in the unmasked portion ofthe backplane being inadvertently removed. The material used to form thefront electrode and the adhesive can be any of the materials used in theprior art for this purpose.

Specific masked backplane processes of the invention will now bedescribed in more detail with reference to FIGS. 1 to 4 of theaccompanying drawings. FIG. 1 is a top plan view of an active matrixbackplane (generally designated 100) having a first, central area 102provided with a two dimensional array of pixel electrodes (not shown),and a second, peripheral area 104 covered by a masking film 106. (Itwill be appreciated that the relative sizes and dispositions of thefirst and second areas 102 and 104 can vary widely and it is notnecessary that the second area 104 surround the first area 102.) Thesecond area 104 will normally include a contact pad for makingelectrical contact with a front electrode (described below) and mayinclude row and column electrodes connected to the pixel electrodes inthe first area 102 and sockets for row and column drivers.

After provision of the masking layer on the backplane, the next step ofthe masked backplane process is coating the backplane with a layer ofelectro-optic material 108, as illustrated in FIG. 2A. Any techniquecapable of depositing the layer of electro-optic material on thebackplane may be used to form layer 108; with an encapsulatedelectrophoretic electro-optic material, slot die coating, bar coatingand spray coating methods have all been successfully used to apply theelectrophoretic material directly to a masked backplane. Depending uponthe deposition method used, the layer of electro-optic material may ormay not cover completely both the first and second areas of thebackplane; for example, some spray coating methods may only coat part ofthe masked area.

The next step of the process is removal of the masking layer 106 toexpose the second area 104 of the backplane 100, and the electricalconnectors and/or sockets thereon, as illustrated in FIG. 2B. As alreadynoted, it is important to choose the masking layer and the electro-opticmaterial such that the masking film and overlying layer of electro-opticmaterial are removed completely from the second area 104 but that noportion of the electro-optic material overlying the first area 102 isremoved.

To complete the assembly of an electro-optic display, it is necessary tosecure a light-transmissive, electrically conductive layer over thelayer of electro-optic material 108. As shown in FIG. 2C, this is mostconveniently effected by providing a front substrate 110 carrying thelight-transmissive, electrically conductive layer or front electrodelayer 112. As discussed for example in the aforementioned U.S. Pat. No.6,982,178, polymeric films coated with indium tin oxide (ITO) areavailable commercially and are very suitable for providing the frontsubstrate 110 and front electrode layer 112. A layer of laminationadhesive 114 is then formed on the front electrode layer 112, leavingexposed a portion of the front electrode layer 112 needed to provide anelectrical connection to the backplane 100. In practice, it is generallymost convenient to coat the whole of the front electrode layer 112 withthe lamination adhesive layer 114 using a roll-to-roll process, cut theresultant roll into portions of the size needed for individual displays,and then to remove or “clean” the lamination adhesive from the portionof the front electrode layer 112 required to provide the electricalconnection. A variety of methods for cleaning the lamination adhesivefrom the requisite portion of the front electrode layer 112 are known inthe art; see, for example, U.S. Pat. No. 7,733,554.

The sub-assembly shown in FIG. 2C is then laminated to the sub-assemblyshown in FIG. 2C with the lamination adhesive layer 114 in contact withthe electro-optic layer 108 to form the final display shown in FIG. 2D.Typically, a conductive ink or similar material 116 is placed on thebackplane 100 adjacent the portion of the front electrode layer 112 notcovered by the adhesive layer 114, as described in the aforementionedU.S. Pat. No. 6,982,178. This final display has the advantage that theelectro-optic layer 108 is in direct contact with the backplane 100(without any intervening adhesive layer), thereby maximizing theresolution of the display. In addition, the positioning of the adhesivelayer 114 adjacent the front electrode layer 112 allows the adhesivelayer 114 to be made highly conductive without detriment to theresolution of the display.

FIGS. 3A-3D illustrate a second masked backplane process of theinvention in which an adhesive layer is formed overlying theelectro-optic layer before the masking layer is removed. The firststages of this second process, namely the provision of a masking layer106 on a backplane 100 and the deposition of an electro-optic layer 108over the backplane, are identical to the first process described above,and result in the sub-assembly shown in FIG. 3A, which is essentiallyidentical to that shown in FIG. 2A. However, the next step in the secondprocess is the coating of a 100 per cent solids radiation-curableadhesive layer 214 over the electro-optic layer 108, to produce thestructure shown in FIG. 3B. The adhesive layer 214 is left uncured atthis step of the process. The masking layer 106 is next removed, asshown in FIG. 3C, thereby removing the portions of both theelectro-optic layer 108 and the adhesive layer 214 previously overlyingthe masking layer 106. Again, it is important to choose the maskinglayer, the electro-optic material and the adhesive such that the maskingfilm and overlying layers of electro-optic material and adhesive areremoved completely from the second area 104 but that no portion of theelectro-optic material overlying the first area 102 is removed.

The final step in the second process is the lamination of a filmcomprising a front substrate 110 and front electrode layer 112 to thesub-assembly shown in FIG. 3C to produce the final display shown in FIG.3D, with the provision of conductive ink 116 or similar conductivematerial as described above with reference to FIG. 2D. Since theradiation-curable adhesive layer 214 is already present in thesub-assembly of FIG. 3C, no further adhesive is needed and the frontsubstrate 110 and front electrode layer 112 can be laminated atsubstantially room temperature and without the use of high pressure. Theuse of the 100% solids adhesive layer 214 allows the front electrodelayer 112 and front substrate 110 to take a variety of forms includingflexible substrates and also rigid substrates like glass. Once the frontelectrode layer 112 and front substrate 110 have been applied, theadhesive layer 214 can be radiation cured with ultraviolet radiation toproduce the final display shown in FIG. 3D. This display, like thatshown in FIG. 2D, has the advantage that the electro-optic layer 108 isin direct contact with the backplane 100 to maximize the resolution ofthe display. In addition, the positioning of the adhesive layer 214adjacent the front electrode layer 112 allows the adhesive layer 214 tobe made highly conductive without detriment to the resolution of thedisplay. The process of FIGS. 3A-3D eliminates the top plane cleaningused in the process of FIGS. 2A-2D, allows for a thinner adhesive layerbecause this layer is applied as a liquid, allows for flexible or rigidfront electrode layers and eliminates the need for a high temperaturelamination step.

FIGS. 4A-4D illustrate a third masked backplane process of the inventionin which two separate masking layers are used and a front electrodelayer is formed directly on the electro-optic layer. The first stages ofthis third process, namely the provision of two separate masking layers106A and 106B on a backplane 100 and the deposition of an electro-opticlayer 108 over the backplane, are generally similar to the first andsecond processes described above, and result in the sub-assembly shownin FIG. 4A, which is generally similar to those shown in FIGS. 2A and 3Aexcept for the provision of the two separate masking layers 106A and106B. Masking layer 106A covers the bonding areas for the driverelectronics and edge seal areas, while masking layer 106B covers thearea for front electrode connection(s). The two masking layers do nothave to be separate films but can be in the form of a single film cut toallow two portions thereof to be removed separately, as illustratedschematically in FIG. 5. Alternatively, depending upon the geometry ofthe areas covered by the first and second masking films, the firstmasking film may cover all areas of the backplane which are not to haveelectro-optic material deposited thereon, and the second masking filmmay be a separate film applied over the first masking film. The maskingfilms previously described can be used. The masked backplane shown inFIG. 5 then has electro-optic material deposited thereon by any of themethods previously described to produce the structure shown in FIG. 4A.

The next step in the second process is removal of the second maskinglayer 106B without removing the first masking layer 106A, thus exposingthe areas of the backplane needed for front electrode contacts, andproducing the structure shown in FIG. 4B. Next, a light-transmissive,electrically-conductive front electrode layer 312 is deposited (normallyby a wet coating process) over the backplane to produce the structureshown in FIG. 4C. The front electrode layer 312 not only forms a frontelectrode over the electro-optic layer 108 but also forms a frontelectrode connection with the exposed areas of the backplane, asillustrated at the right hand side of FIG. 4C. The front electrode layer312 may be formed from a conductive polymer, for examplepoly(3,4-ethylenedioxythiophene) (“PEDOT”), normally used in the form ofits poly(styrenesulfonate) salt (“PEDOT:PSS”) or a polyaniline, or maybe formed from network of conductors, for example carbon nanotubes ornanowires. The present inventors have successfully coated both PEDOT andcarbon nanotube front electrodes directly on an encapsulatedelectrophoretic layer.

The final step of the process is removal of the first masking layer106A, together with the overlying portions of the electro-optic layer108 and the front conductor layer 312 to produce the display illustratedin FIG. 4D. If desired driver electronics and/or edge seals may now beplaced in the exposed areas of the backplane.

The display shown in FIG. 4D, like those shown in FIGS. 2D and 3D, hasthe advantage that the electro-optic layer is in direct contact with thebackplane, thus maximizing the resolution of the display. However, incontrast to the displays described above, the display shown in FIG. 4Dhas no lamination adhesive between its electrodes, thus completelyeliminating the electrical effect of such adhesive. Thus, the displaystructure shown in FIG. 4D enables the highest resolution andtemperature performance for a given electro-optic layer. One potentialpractical problem with the display structure shown in FIG. 4D is thatany pore or pinhole in the electro-optic layer would allow the coatedfront electrode to come into electrical contact with the pixelelectrodes on the backplane, thus shorting the display.

From the foregoing, it will be seen that the masked backplane process ofthe present invention can provide high resolution addressing withoutcompromising temperature performance, thus removing the limitationsimposed by prior art display construction methods which require a thinadhesive between the electro-optic layer and the backplane. Additionallythe masked backplane process opens up the possibility of conducting theentire manufacturing process in a single fab.

Spray Coating Process

As already mentioned, this invention also provides a process forspraying capsules of an electrophoretic medium on to a substrate. Thisprocess comprises forming a dispersion of the capsules in a liquid;feeding the dispersion through a first orifice; and feeding a continuousstream of gas through a second, annular orifice surrounding the firstorifice, thereby forming a spray of the capsules. This spray coatingprocess has the advantage over slot coating that spray coating normallydoes not require the use of rheology modifiers in the liquid beingsprayed, so that the final coating is free from such rheology modifiersand hence free from the effects such rheology modifiers may have uponthe properties of slot coated electrophoretic media. Typically, in spraycoating, only the additives actually needed in the final product need beadded to the liquid being sprayed.

FIG. 6 is a schematic cross-section through a simple spray coatingnozzle (generally designated 600) which may be used in the spray coatingprocess of the present invention. The nozzle 600 comprises asubstantially cylindrical body 602 having a central, axial bore 604through which is pumped electrophoretic capsules (not shown) dispersedin a liquid (also not shown). The central bore 604 is surrounded by anannular bore 606, through which is forced a continuous stream of air.The lower end of the central bore 604 terminates in an orifice 608,which the lower end of the annular bore 606 terminates in an annularorifice 610, which surrounds orifice 608. A cylindrical baffle 612surrounds the annular orifice 610. The air flow through the annularorifice 610 constrained by the baffle 612 causes the dispersion ofcapsules passing through orifice 608 to form a spray or jet 614.

The nozzle 600 is also provided with shaping air bores 616, which may besix or eight in number. As shown in FIG. 6, the peripheral portions ofthe nozzle 600, through which the bores 616 pass, extend downwardlybelow the orifices 608 and 610 and the baffle 612, and the lowerportions of the bores 616 are directly downwardly and inwardly. Shapingair is forced continuously through the bores 616 so that it impinges onthe jet 614, thereby causing the jet to open out into a wide spray 618,which impinges on a substrate 620 disposed below the nozzle 600.

FIGS. 7 and 8 illustrate a high-volume low-pressure atomization nozzle(generally designated 700) suitable for use in a high volume spraycoating process of the present invention. It will be appreciated that inuse the nozzle 700 would normally be inverted relative to the positionillustrated in FIGS. 7 and 8 so that capsules emerging from the nozzlewould be directed downwardly on to a substrate, as illustrated in FIG.6.

As will readily be apparent to those familiar with spray nozzletechnology, the nozzle 700 shown in FIGS. 7 and 8 operates insubstantially the same manner as the nozzle 600 shown in FIG. 6 but thenozzle 700 has the following structural differences:

-   -   (a) the central axial bore 702, through which the capsule        dispersion passes, tapers adjacent its orifice 708 to increase        the velocity at which the capsule dispersion passes through this        orifice;    -   (b) the simple annular bore 606 in FIG. 6 is replaced by a        combination of air feed passage 706A, a cylindrical air plenum        706B, a conical tapering portion 706C and an outlet portion 706D        (FIG. 8) which terminates in an annular orifice 710; in        addition, auxiliary shaping air jets are provided by bores 706E        which extend from the tapering portion 706C to orifices on        either side of the orifice 708; and    -   (c) the shaping air bores 716 are provided with double outlet        portions 716A, 716B to provide greater control of the shape of        the final capsule spray.

FIG. 8 shows four dimensions which have been found important inachieving good spray coating results from the nozzle 700 shown in FIGS.7 and 8, these four dimensions being (A) the radius of the centralorifice 708; (B) the radial distance between the outer edge of thecentral orifice 708 and the inner edge of the annular orifice 706D; (C)the radial width of the annular orifice 706D; and (D) the axial distancebetween the orifices 708 and 710.

The quality of capsules coatings is assessed in terms of theirreproducibility granularity, mean coating weight, uniformity and defectdensity; defect density is quantified by the number of non-switchingcapsules per unit display area in a standard display structure, whichfor present purposes is defined as a backplane bearing, in order, a 25μm layer of lamination adhesive, a 20 μm capsule layer and a frontsubstrate comprising an ITO layer on 25 μm polyethylene terephthalatefilm. The first factor to be considered in achieving good spray coatingsis capsule and gas flow rates and pressures. It has been foundempirically that capsule spraying is best achieved using a high-volume,low-pressure (“HVLP”) nozzle; a variety of standard nozzle designs knownin the art may be used, but the preferred design is that shown in FIGS.7 and 8. Preferably, the ratio of atomization air outlet cross-sectionto capsule dispersion outlet cross section is not greater than about8.5, and preferably between about 5.0 and about 7.0. The capsuledispersion orifice diameter (twice A in FIG. 8) is preferably in therange of about 1.0-1.40 mm. The capsule dispersion may contain capsulesin a weight fraction preferably between about 38.0 and about 40.5 weightper cent; this dispersion may optionally contain 1-butanol at aconcentration of up to about 4.0 weight per cent and a surfactant, suchas Triton X-100 at a concentration of up to about 0.04 weight per cent.

A wide range of capsule dispersion feed rates and atomization air feedrates can be used in the spray coating process of the present invention.Typically, the capsule dispersion feed rate, M_(F), is not less thanabout 30 g/min and not greater than about 70 g/min, the optimum beingdetermined mainly on the basis of an appropriate residence time in theatomization zone, that is to say the region in which the capsuledispersion column emerging from the first orifice breaks into sheets offluid, which subsequently break into ligaments and finally droplets.Desirably, the droplet size distribution is such that the mean capsulecount per droplet is less than about 5.0, and the standard deviation isless than about 3.0, capsules per droplet. The atomization air feed rateis set on the basis of a critical air velocity, v*, measured at thesecond orifice, and is typically of the order of about 100 m/sec. In thepreferred process, a total air feed rate, M_(A), (including atomizationair and shaping air) of approximately 150 to 200 g/min is employed inthe absence of shaping air, and up to 300 g/min with shaping air.

Empirically, it has been found that the operating window for HVLPatomization in terms of M_(A)/M_(F) versus M_(F), has the form shown inFIG. 9, although the numerical values involved will vary with theparticular nozzle design used. The unshaded region of the graph of FIG.9 represents the desirable operating window. The shaded regionsrepresent defect regions which result in undesirable spray patterns suchas excessive fluid velocity (“jetting”), highly irregular and transientspray structure, and coarse droplet distribution.

In the spray coating process of the present invention, the air feed rateand nozzle-to-substrate distance should be carefully controlled to avoidcapsule damage. In general, a nozzle-to-substrate distance of 200 to 320mm is optimal, and this distance should be adjusted approximatelyinversely to atomization air velocity squared.

It has also been found that the quality and uniformity of the sprayedcapsule coating can be strongly influenced by pretreatment of thesubstrate and by additives added to the capsule dispersion. Usefulpretreatments and additives include but are not limited to:

1) Capsule dispersions that incorporate surfactants such as TritonX-100, butanol etc. to improve wetting of the substrate surface;

2) Pre-coating of the substrate surface with sub-layers incorporatingsurfactants such as Triton X-100, 1-butanol, and others possessing adetergent structure, and optionally a polyurethane latex;

3) Pre-treating the substrate with an atmospheric plasma or coronadischarge treatment; and

4) The capsule dispersion may contain polymeric binders, for example apolyurethane latex

Example 1

A capsule dispersion was sprayed with an HVLP nozzle using inletatomization air at a pressure of 20 psig (about 330 MNw m⁻²) measured atthe nozzle inlet, with a gravity feed of the capsule dispersion.Depending on the dispersion viscosity, the mass flow rate of thedispersion was approximately 25 to 35 g/min. The spray was directedvertically downward and deposition took place at near normal incidenceon to a horizontal substrate so as to avoid inclined plane flow afterdeposition. The nozzle-to-substrate distance was 240 to 280 mm, but maybe lower or higher. Capsule spraying took plane across a thin filmtransistor backplane in one or more passes to achieve a target meancoating weight given by the following relationship:

$\Theta = {{M_{F}\frac{\eta}{Wv}{Nm}\frac{dv}{dt}} = {F^{B} + F^{H} + F^{P}}}$

where Θ is the mean coating weight (in g/m²), M_(F) is the dispersionmass feed rate (in g/min), N is the number of passes over the substrate,η is the spray transfer efficiency at each pass (which should be atleast 50%), W is the substrate width (in meters), v is the actuationvelocity (in m/min). In one process of the invention, the target meancoat weight Θ=20-d, M_(F)=35 g/min, η˜60˜70%, and W=0.107 m. In thisprocess, multiple coating passes could be used so long as the totalresidence time of a given substrate underneath the nozzle did not exceedabout 3 or 4 seconds; longer coating times left the thin sub-layersineffective by evaporation.

As already mentioned, the spray coating process of the invention mayinclude the use of a masking material covering part of the substrate sothat, after removal of the masking material, capsules remain only onthose portions of the substrate where the masking material was notpresent. The masking material used to cover part of the substrate shouldnot be porous, or at least should have low enough porosity to ensurethat capsule deposition on to the masked areas of the substrate does notoccur. The masking material should not significantly absorb the liquid(usually aqueous) in which the capsules are dispersed, and should beplaced close enough to the surface of the substrate that lateral draftof capsules beneath the masking material from the unmasked regions ofthe substrate into the masked areas does not occur. After the capsuleshave been deposited on the substrate, the capsules may be dried (orotherwise treated to form a coherent layer, for example by exposure toradiation) with the masking material still in position, or the maskingmaterial may first be removed and then the capsules dried or otherwisetreated. In either case, the physical properties of the masking materialand the capsule dispersion should be chosen so that, during the removalof the masking material, capsules are not dragged into previously maskedareas of the substrate, nor are capsules removed from unmasked areas(for example, by irregular tearing of a coherent dried layer ofcapsules.

The masking film may comprise an adhesive pre-laminated on to thesurface on to which the capsules are to be deposited, and a release filmexposed to the spray. After capsule deposition, the release film isremoved, followed by additional processing. The resultant spray-printedfilm may then be laminated to a backplane, which may be eithertransparent or opaque.

FIG. 10 is a top plan view of a first electrophoretic produced by amasked spray coating process of the invention. The backplane is madetransparent to allow visibility through the display outside of the areas(circles in FIG. 10) on which the capsules have been deposited. Such abackplane can generate a patterned image with as many individual opticalstates as the electrophoretic medium is capable of generating. In thedisplay shown in FIG. 10, the capsules contain a white and a magentapigment, so that all possible states of the display are a combination ofmagenta and/or white, including the extreme magenta and white opticalstates.

As already mentioned, the masked spray coating process of the inventionmay comprise more than one coating step and thus allow deposition of twoor more different electrophoretic media on a single substrate. FIG. 11is a top plan view of a display produced in this manner. The displayshown in FIG. 11 is produced by applying a first masking material to afront plane electrode, and then spray coating electrophoretic capsulescontaining blue and white pigments over the first masking material.After drying the capsules and removing the first masking material, asecond masking material is applied to the front electrode, andelectrophoretic capsules containing yellow and white pigments are spraycoated on to the electrode. The second masking material is then removedand the front electrode and overlying electrophoretic layer laminated toa backplane. The display shown in FIG. 11 has two primary opticalstates, namely a uniform color determined by the common pigment (in thiscase, white) and a second patterned (blue/yellow) state as illustratedin FIG. 11.

The spray coating process of the present invention overcomes thelimitation of prior art coating processes such as slot coating and thusprovides the ability to do patch coating and coating of threedimensional objects. The spray coating process is also less susceptibleto streaking due to clogging of the die in slot coating processes, andthus can offer enhanced yields. The spray coating process also avoidsthe need for lamination adhesive layers between the electrodes of adisplay, thus permitting a higher electric field across theelectrophoretic layer for a given operating voltage and thus enablinghigher white state brightness and higher contrast ratio, as well as thepotential for reduced blooming and enhanced microcontrast as a result ofthe electrophoretic capsules being in direct contact with the backplane.

Printed encapsulated electrophoretic displays are desirable inapplications such as window screens, wall panels, or other architecturalelements in which either a low information density display or artisticrelief is desired with minimal or no active matrix driving. Instead, theinterfaces between switching and non-switching, or between two regionsof differing electrophoretic medium which switch in a qualitativelydifferent manner, may be pre-patterned into the display. The maskedspray coating process of the present invention provides a way ofachieving these aims without compromising the mechanical integrity ofthe deposited capsules.

Swellable Polymer Coating Process

As already mentioned, this invention provides a process for forming amonolayer of capsules on a substrate, the process comprising depositinga solution of a water swellable polymer on the substrate; and thereafterdepositing a quantity of the capsules sufficient to form a monolayer ofcapsules on to the substrate, and allowing the capsules to arrangethemselves into a monolayer on the substrate.

In this process, it is important to control the quantity of capsulesdeposited on each unit area of the substrate; this quantity should becontrolled so that the capsules can rearrange on the substrate into atightly packed monolayer. The rearrangement of capsules may take placeimmediately after the deposition of the capsules on the substrate but,perhaps more commonly, may take place after the capsule layer is driedor otherwise treated to form a coherent layer of capsules on thesubstrate. As previously noted, it is desirable that the deposition ofthe capsules be effected by a process such as spray coating (oralternatively curtain coating or deposition of capsules from nozzles orsimilar processes) which do not require contact of a coating head or barwith the capsule layer. With the swellable polymer reducing adhesion ofthe capsules to the substrate, a coating head or bar will tend to dragthe capsules along with it, thus resulting in a very sparse capsulecoating on the substrate, too sparse to enable a well packed monolayerof capsules to be formed. If coating is attempted without the swellablepolymer pre-treatment, the capsules stick to the substrate as they aredeposited by a coating head or bar, but the adhesion of the capsules tothe substrate is so great that capillary forces are insufficient toallow for capsule rearrangement and the formation of a well packedmonolayer of capsules.

FIGS. 12A to 12H of the accompanying drawings illustrate successivestages of an experimental process of the present invention in which amicroscope slide was treated with egg albumen and then had capsulesdeposited thereon from a pipette. It will be seen from these Figuresthat the originally scattered capsules deposited from the pipette weregradually drawn by capillary forces into a closely packed monolayercovering about three-fourths of the area of the slide shown in theFigures. When a similar experiment was attempted with bar coating on analbumin-treated slide, the capsules simply clung to the coating bar andvirtually no capsules were left behind on the slide. When covering muchlarger areas than a microscope slide, the albumin coating process of thepresent invention renders it possible to generate large area of closelypacked monolayers of capsules.

From the foregoing, it will be seen that the swellable polymer coatingprocess of the present invention provides a process for generating largequantities of closely packed capsule monolayer coatings usingconventional equipment and materials suitable for mass production. Theprocess should produce coatings essentially free from grain, especiallyif the capsules are applied by spray coating, as the spray shouldrandomize the size distribution of capsules over the coating area. Theswellable polymer coating process may be especially useful in providingcoatings for use in variable transmission windows, where multilayers ofcoatings and coating defects (uncoated areas) are highly visible andadversely affect the quality of the windows.

Overcoat Layer Process

As already mentioned, the overcoat layer process of this inventioncomprises: providing a backplane comprising at least one electrode;coating the backplane with a layer of an electro-optic material;depositing a layer of a substantially solvent-free polymerizable liquidmaterial over the layer of electro-optic material; contacting thepolymerizable liquid material with at least one light-transmissiveelectrically-conductive layer; and exposing the polymerizable liquidmaterial to conditions effective to cause polymerization of thematerial, thereby adhering the at least one light-transmissiveelectrically-conductive layer to the layer of electro-optic material.

The advantages of the overcoat layer process of the present inventionmay be seen by comparing FIGS. 13 and 14. FIG. 13 shows a schematiccross-section through a prior art color display (generally designated1300) comprising, in order, a backplane 1302, a first (relatively thin)adhesive layer 1304, a monochrome electro-optic layer 1306, a secondadhesive layer 1308 substantially thicker than the first adhesive layer1304, a front electrode layer 1310, a front substrate 1312 and a colorfilter array 1314, which may be printed directly on to the frontsubstrate 1312. This structure may be formed using a double release filmin the manner described above. Note that in the display 1300 the CFA1314 is separated from the electro-optic layer 1306 by the thicknessesof the front substrate 1312 and the second adhesive layer 1308, whichtogether are typically about 50 μm thick. (The front electrode layer1310 is typically less than 1 μm thick and thus for practical purposesits thickness may be ignored.)

FIG. 14 is a schematic cross-section, similar to that of FIG. 13, buttaken through a display (generally designated 1400) produced by theovercoat layer process of the present invention. The backplane 1302,electro-optic layer 1306, front electrode layer 1310, front substrate1312 and CFA 1314 are all similar to the corresponding layers in theprior art display 1300 shown in FIG. 13. However, in FIG. 14 theelectro-optic layer 1306 is coated directly on to the backplane 1302 sothat the first adhesive layer 1304 present in the display 1300 iseliminated. Furthermore, the second adhesive layer 1308 in display 1300is replaced in display 1400 by a much thinner adhesive layer 1408 formedby polymerization of a solvent-free polymerizable liquid material. Theadhesive layer 1408 will typically have a thickness of only about 5 μm,thus reducing the spacing between the CFA 1314 and the electro-opticlayer 1306 to about 30 μm, a 40 per cent reduction from the spacing indisplay 1300, with a corresponding reduction in both illumination andviewing parallax, thus providing a wider viewing angle and higher colorsaturation. In addition, the elimination from display 1400 of the firstadhesive layer 1304 in display 1300 increases the voltage drop acrossthe electro-optic layer and reduces blooming. The overcoat layer processof the present invention may also be applied to black-and-white displayshaving a structure similar to that of display 1400 but lacking the CFA1314.

As already indicated, the present invention may also be applied toinherent color displays, as illustrated in FIGS. 15 and 16. FIG. 15 is aschematic cross-section through a prior art display (generallydesignated 1500), which is generally similar to the display 1300 shownin FIG. 13 except that the electro-optic layer 1506 is an inherent colorelectro-optic layer which is capable of displaying a range of colors atevery pixel of the display, as described, for example in U.S. Pat. No.8,576,476, and the color filter array is omitted. FIG. 16 shows adisplay 1600 produced by the overcoat layer process of the presentinvention. As with the display 1400 described above, in the display 1600the first adhesive layer 1304 is omitted and the second adhesive layer1308 is replaced with a much thinner adhesive layer 1408 formed bypolymerization of a solvent-free polymerizable liquid material. As withthe display 1400 described above, eliminating the first adhesive layer1304 allows more of the electric field applied to the display to residewithin the electro-optic layer 1506, resulting in a brighter white stateand higher contrast ratio. In addition, micro contrast will be greatlyimproved because blooming effects associated with the first adhesivewill be eliminated, thereby increasing color gamut and image sharpness.

The overcoat layer process of the present invention may include variousoptional features. When the electro-optic layer is to be an encapsulatedelectrophoretic layer, the capsule slurry used to apply the capsules tothe backplane may include surfactants, such as Triton X-100 or butanol,to improve wetting of the backplane. Prior to coating of theelectro-optic layer, the backplane may be pre-coated with surfactants,such as Triton X-100 or butanol, or with a polyurethane latex.Alternatively or in addition, the backplane may be pre-treated withplasma (including atmospheric plasma) or corona discharge treatment.Such treatment may be effected at various power settings and withvarious gases, including but not limited to oxygen, nitrogen etc. Aspreviously noted, in general it is preferred that the electro-opticlayer in the overcoat layer process be applied by spray coating,including electrostatic spray coating, but other application techniques,such as slot die coating, blade coating and roll coating (includingflexo and gravure techniques) may also be used. When the electro-opticlayer is to be an encapsulated electrophoretic layer, the capsules aredesirably in the form of a slurry containing a polymeric binder, forexample a polyurethane latex.

Example 2

An overcoat layer process of the present invention was carried out bydepositing capsules of an electrophoretic medium on to a backplane,overcoating the capsules with a solvent-free polymerizable liquidmaterial and adhering a front electrode layer/front substrate (in theform of a poly(ethylene terephthalate) film coated on one surface withITO) to the electrophoretic medium by means of the polymerizable liquidmaterial.

The spraying of the capsules on to an active matrix backplane waseffected using the spray coating process of the present invention andsubstantially as in Example 1 above using an HVLP nozzle at a pressureof 20 psig (about 330 MNw m⁻²) measured at the nozzle inlet, with agravity feed of the capsule dispersion at a mass flow rate of 25 to 35g/min. The spray was directed vertically downward and deposition tookplace at near normal incidence on to a horizontal substrate so as toavoid inclined plane flow after deposition. The nozzle-to-substratedistance was 240 to 280 mm. The target coating weight was 20 g/m².Multiple spray heads and higher dilution coating slurries may contributeto increased coating uniformity.

The polymerizable liquid material used was formulated as follows (thevarious Sartomer resins used are available from Sartomer Americas, Inc.,Overland Park Kans.):

-   -   Sartomer SR 9087 44.55% by weight    -   Sartomer SR 9038 14.85% by weight    -   Sartomer CN 3108 39.6% by weight    -   TPO diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide 0.5% by        wt.    -   1-Hydroxycyclohexyl phenyl ketone 0.5% by weight.        These components were combined and placed on a roll mill for at        least eight hours to ensure thorough mixing.

The displays were assembled as follows. A metal pan was covered withcardboard to provide cushioning, and a sheet of plastic release sheetwas placed on top of the cardboard. The capsule-coated backplane wasplaced on this pan, and polyimide tape was used to cover the contacts onthe backplane. A PET/ITO film was cut to the size of the backplane,placed over the capsule-coated backplane, and taped in place withpolyimide tape. A sheet of metalized release sheet was placed on top ofthe stack, and the whole assembly moved to a laminator, with the rollerclosed just barely on the glass of the backplane. The laminator was setto 20 psi and 25 ft/min (7.62 m/min) to assure an ultraviolet-curedcoating of the desired thickness. The PET/ITO was lifted up, allowing abead of the polymerizable liquid mixture to be placed as close aspossible to one edge of the PET/ITO film; the film was lifted for aslong as possible while the roller moves the polymerizable liquid mixtureto the opposed edge of the backplane. Finally, the metalized releasefilm was removed and the polymerizable liquid mixture cured. The tapeused to cover the contacts on the backplane was removed, and use carbontape (or silver paste) applied to make electrical contact with the ITOlayer. The display thus produced was conditioned at 25° C. and 50%relative humidity for 5 days, then edge sealed with a hydrophobic UVcurable polymer.

From the foregoing, it will be seen that the overcoat layer process ofthe present invention enables higher white state brightness in bothcolor and monochrome displays, and increased color gamut in bothinherent color and color filter array displays.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A process for forming a monolayer of capsules on a substrate, theprocess comprising: depositing a solution of a water-swellable polymeron the substrate; and thereafter depositing a quantity of the capsulessufficient to form a monolayer of capsules on to the substrate, andallowing the capsules to arrange themselves into a monolayer on thesubstrate.
 2. A process according to claim 1 wherein the water-swellablepolymer comprises at least one of a polysaccharide and a protein.
 3. Aprocess according to claim 2 wherein the polymer comprises at least oneof a pectin or an albumin.
 4. A process according to claim 3 wherein thepolymer comprises egg albumin or bovine serum albumin
 5. A processaccording to claim 1 wherein the capsules are formed of a hydrophilicmaterial and the polymer solution is aqueous.
 6. A process according toclaim 1 wherein the deposition of the capsules is effected by spraycoating.